In the hospital setting, mobile radiographic exams are performed on patients that are incapable of being moved, or are difficult to move. In tertiary care medical centers, mobile radiographic exams represent a significant percentage of the radiographic exams performed. X-rays passing through an object, such as a human body, experience some degree of scatter associated with interactions with atoms or electrons. The primary x-rays transmitted through an object travel on a straight line path from the x-ray source (also referred to herein as the x-ray focal spot) to the image receptor and carry object density information. Scattered x-rays form a diffuse image that degrades primary x-ray image contrast. In thick patients, scattered x-ray intensity exceeds the intensity of primary x-rays. Scattering phenomena is well known and routinely compensated for in general radiography, fluoroscopy and mammography through the use of anti-scatter grids.
An anti-scatter grid includes a laminate of lead foil strips interspersed with strips of radiolucent material (FIG. 1). The grid is positioned between the object of interest and the x-ray image receptor plate and oriented such that the image forming primary x-rays are incident only with the edges of the lead foil strips. Thus, the majority of primary x-rays pass through the radiolucent spacer strips. In contrast, scattered x-rays are emitted in all directions after interaction with the object and as such, scattered x-rays are incident on a larger area of the lead strips and only a small percentage of scattered x-rays are transmitted by the grid, as compared to primary x-rays. The degree of scatter control for a given grid depends upon the grid ratio, which is defined as the ratio of the radiopaque strip thickness in the direction of the x-ray path to the width of the radiolucent spacer material as measured orthogonal to the x-ray beam path. Thus, the higher the grid ratio, the greater the scatter control. A high grid ratio, while more effective, is also more difficult to align relative to a focal spot. In order to compensate for x-ray beam divergence in a focused grid, the radiopaque strips are tilted to a greater extent with increasing distance from the center of the grid. The planes of the grid vanes all converge along a line known as the focal line. The distance from the focal line to the surface of the grid is referred to as the focal length of the grid. The focal line coincides with the straight line path to the focal spot (illustrated in FIG. 2). Thus, when the focal spot is coincident with the focal line of the grid, the primary x-rays have minimal interaction with the radiopaque lead strips and maximal primary transmission is obtained. Misalignment of the focal line of the anti-scatter grid with the focal spot diminishes primary x-ray transmission while scattered x-ray transmission remains unchanged. Thus, optimal primary x-ray transmission requires alignment (positional and orientational) of the focal spot with the focal line of the anti-scatter grid.
In general radiography, fluoroscopy and mammography, the image receptor and x-ray tube are rigidly mounted and in a fixed position relative to one another, thereby making focal spot and grid alignment a simple process. In mobile radiography, an image receptor is placed under a bedridden patient and the x-ray source is positioned above the patient. Since the relative separation of the focal spot and the image receptor is variable, determining the proper position and orientation of an anti-scatter grid between a patient and the image receptor becomes a difficult alignment problem. If a grid is not used, only a small fraction of the possible contrast is obtained in the x-ray image. As a result, scatter to primary x-ray ratios of 10:1 or more are common in chest and abdominal bedside radiography resulting in less than 10% of the possible image contrast being obtained in mobile radiographic films ([1,2]Barnes, G T, RadioGraphics 11:307-323, 1991; Niklason et al., Med. Phys. 8:677-681, 1981). Contrast limitations are exacerbated if digital storage phosphor image receptors are utilized in place of the more conventional screen-film systems ([3]Tucker et al., Radiology 188:271-274, 1993).
When grids are utilized in conjunction with mobile radiography, the grid is typically not aligned. Misalignment problems are diminished by utilizing a grid having a low ratio of 8:1 or less. Although x-ray image contrast is improved with the use of a low ratio grid, the contrast remains significantly lower than otherwise could be obtained with a properly aligned, high ratio grid having a grid ratio of 10:1 or greater.
Thus while mobile radiography is in many ways more convenient than fixed installation radiography, its clinical utility is diminished due to the inferior image quality caused by scattered radiation which is a greater problem in mobile radiography due to the difficulty in producing the proper alignment of the focal spot with the anti-scattering grids. A means to produce proper alignment that is easy for the operator to use would significantly improve mobile radiographic image contrast and image quality, and thus increase the clinical utility of mobile radiography.
A system is disclosed in U.S. Pat. No. 4,752,948 which includes a rigid arm mounted on the grid tunnel, with a coupling on the other end that connected to the x-ray source housing. A hinge on the grid end of the arm allowed it to be folded for transportation. A radiography technologist unfolds the arm, locks the hinge, slides the grid tunnel and film cassette under the patient, and attaches the x-ray source housing to the other end of the arm. The arm then holds the x-ray source rigidly in alignment with the grid tunnel. This system demonstrated the image quality and clinical advantages of employing properly aligned high ratio grids in bedside radiography. However, difficulty in using the system limited the application thereof in mobile radiography.
Loren Niklason et al. disclosed a mobile radiography system utilizing a telescoping arm ([4]Niklason et al., Radiology 173(P):452, 1989). One end of the arm was permanently attached to the mobile x-ray unit column and the other end was attached by the radiography technologist to the grid assembly after the grid and cassette were positioned under the patient and the mobile unit was centered right-to-left to the grid assembly. Dials indicated to the technologist the transverse direction the tube needed to be moved and the angle the tube had to be rotated to align it with the grid. The time consuming and complex steps to align the x-ray tube using this system limited the application thereof in mobile radiography.
U.S. Pat. Nos. 5,241,578 and 5,388,143 disclose a laser alignment device that required a user to align a laser and a mark in the alignment light field with a reflector device that mounted on a corner of a grid tunnel. As with Niklason""s system, this system required the user to manually align the x-ray source by trial and error. Further, it required that part of the grid tunnel extend past the patient, which limited the application thereof in portable radiography.
Peter O""Donovan et al. disclosed a system involving electronic levels on the grid tunnel and source housing, an alignment target attached to the source, and crosshairs in the alignment light field ([5]O""Donovan et al., Radiology 184:284-285, 1992). A tape measure was used to ensure that the source was the proper distance from the grid tunnel. The user rotated the source housing until the two levels indicated that the central axis of the source was normal to the grid tunnel in one direction; turn on the collimator light; and move the tube housing until the shadow of one of the cross-hairs fell on a mark on the grid tunnel. The complexity of this procedure limited the application thereof in mobile radiography.
The prior art systems have been limited in their utility in clinical acceptability owing to the considerable additional effort required on the part of a radiography technologist to align the x-ray source. Thus, there exists a need for a mobile radiography system having a simple means to place the focal spot and the central x-ray beam in correct alignment (position and orientation) with regard to the anti-scattering grid.